In recent times there has been considerable development and increase in the use of plastic materials for example in the building industry, the refrigerating industry, in the production of unsinkable members e.g. for boats, in packaging and other product conditioning, and the aeronautical and automobile industry. One consequence of this increased use has been a certain amount of progress in improving apparatus and processes for producing expanded plastic material. For example, one step along this route was the use of a high-frequency field to melt the synthetic resin, as disclosed for example in French Pat. Nos. 2 149 529, 1 217 351, 2 045 888 and 2 186 344, and U.S. Pat. No. 3,377,653.
A further improvement lay in the use of ultra-high frequency or microwave radiation (generally 500 to 300,000 MHz) which made it possible to produce plastics foams in an economical manner. This process does not require a capacative circuit and does not cause voltage cracking, the ultra-high frequency (referred to herein as U.H.F.) field energizing a resonant cavity or a resonant waveguide. A process of this nature is disclosed in French Patent applications Nos. 76 01049 and 76 31899 in the name of the assignees of the present application, which set forth an operating procedure of simultaneously introducing beads or pearls of plastic materials which had first been pre-expanded, and water, into a mold disposed in a resonant cavity. This mode of operation produces conditions which permit the formation of vapour in situ, and excellent welding of the beads or pearls to each other, with a good degree of distribution of the heat involved in melting the material.
However, this process suffers from the disadvantage that the molds used are of a material which is transparent to or transmissive of the U.H.F. radiation or wich has a low absorption capacity in respect of such radiation. A consequence of this is a certain lack of uniformity in the heating action and an increase in the amount of power which has to be employed due to the fact that it is necessary to heat the walls of the mold in each molding cycle to achieve heating uniformity. This process therefore not only suffers from poor power efficiency but also tends to suffer from an extended operating cycle due to the need for additional heating of the mold.